1. Field of the Invention
This invention relates to an acrylic synthetic fiber, especially an acrylic synthetic fiber having vein-like or straw-like voids extending substantially in parallel to the longitudinal axis of the fiber, which can be easily split into finer fibers by an external force, and a process for the preparation of this acrylic synthetic fiber. Furthermore, the present invention relates to a pulpy acrylic synthetic fibrous article having excellent properties as the starting material for a friction material, paper or nonwoven fabric. Moreover, the present invention relates to a friction material comprising this pulpy acrylic synthetic fibrous article as a base material.
2. Description of the Related Art
Hollow acrylic fibers are known, for example, from Japanese Unexamined Patent Publication No. 51-149922 and Japanese Unexamined Patent Publication No. 57-89612. The conventional hollow acrylic fibers include a fiber having cell-like independent voids in the interior thereof and a tubular fiber having in the interior thereof a hole continuous along the fiber axis.
The hollow acrylic fiber having cell-like independent voids has only a few large voids in the transverse section thereof, as disclosed in, for example, Japanese Unexamined Patent Publication No. 51-149922.
The hollow acrylic fiber disclosed in Japanese Unexamined Patent Publication No. 57-89612 has in the transverse section thereof several of relatively large voids, as shown in the drawings of the patent publication, and this fiber cannot be easily split.
Voids in these known hollow fibers are cell-like voids or long voids extending along the longitudinal direction of the fiber, and the length of these voids is about 40 to about 50 .mu.m at most.
The objects of forming voids in fibers in the conventional techniques are to decrease the weight, improve the heat-insulating property, impart a water-absorbing property, give a soft touch, and give a dry touch. To attain these objects, acrylic synthetic fibers having voids as disclosed in the above-mentioned patent publications provide excellent results.
Recently, the need for a fiber having a variety of greatly improved properties has increased, and attention is now focused on a fiber having characteristics such that, after a formation of a fiber structure or after a further formation of the fiber structure into a fibrous product such as a knitted or woven fabric, the fiber can be split into finer fibers by various means. Fibers having such properties are characterized in that the freedom of processability is increased, whereby the fibers can be split into finer fibers at an optional processing stage after a formation of fiber structures, and fibrous products having excellent properties not attainable from conventional fibers can be provided.
From this viewpoint, the hollow acrylic synthetic fibers disclosed in the above-mentioned patent publications have problems in that they cannot be split into finer fibers by an external force, for example, by beating and rubbing.
The main reasons why the fibers disclosed in the above-mentioned patent publications cannot be easily split into finer fibers by an external force are that (1) the proportion of voids in the transverse section of fiber is small and (2) the voids are cell-like voids and do not extend far in the longitudinal direction of the fiber.
Fibers that can be split into finer fibers by an external force are known from, for example, Japanese Unexamined Patent Publication No. 47-32122 and Japanese Unexamined Patent Publication No. 55-30460.
The fiber disclosed in Japanese Unexamined Patent Publication No. 47-32122 is a conjugate fiber in which, in the transverse section of a single filament, a water-insoluble polymer is separated into several parts by a water-soluble polyamide extending in radial directions. The fiber disclosed in Japanese Unexamined Patent Publication No. 55-30460 is a fibrilated conjugate fiber composed of a polyamide and a polymer having no affinity with the polyamide.
The costs of these fibers are inevitably high, mainly for the following reasons. Namely, since the fibers are formed by bonding at least two polymers having different characteristics, different polymers must be used, and a special spinneret must be used for the conjugation. Moreover, it is difficult to maintain a constant ratio between the two components, and when both components are made finer and bonded together, an advanced technique is necessary for adjusting the ratio between the two components.
Prior to the present invention, a fiber composed of an acrylic polymer, that can be easily split into finer fibers by an external force, was not known.